1. Field of the Invention
This invention is directed to a classifier having a wash region for cleaning particulate solids prior to further processing or disposal and may be associated with a slaking unit in a slaking machine.
The classifier was developed early in the century for minerals classification, but it has since been successfully employed to produce a consistently high grade of milk of lime for use in neutralizing acid waste streams, in flue gas desulfurization in the power industry, and for preparation of flotation slurries in mineral processing. These prior art machines consist of a rectangular tank having an inclined deck or ramp, a conveyor structure within the rectangular tank which runs parallel to and is positioned close to the inclined deck of the tank for moving settled particulate material or grit along the inclined deck to a point above the liquid or slurry level in the tank. The classifier may include a wash mechanism for washing the grit near the upper end of the inclined deck.
The feed material or slurry to be treated is fed into the lower end of the tank. The liquid in the tank only partially fills it, with the deepest part of the liquid at the lower end of the tank. An outlet is provided for clarified liquor through the end or sidewall of the tank to remove the liquor and control the liquid level in the tank. In operation, then, the lower portion of the inclined conveyor mechanism is below the liquid level in the tank, while the upper portion of the conveyor mechanism and inclined deck are above the liquid in the tank.
The conveyor system :transports grit up the inclined deck of the tank, usually through a wash zone above the liquid level of the tank in a so-called "beach" portion of the deck or ramp surface. Grit particles are drained of associated liquid and of wash liquid in this "beach" portion before being discharged from the tank at its upper end. The drained liquid flows back down the ramp into the liquid in the tank.
In the Kraft or sulfate process of the pulp and paper industry, the pulp is prepared by digesting wood chips in a strongly alkaline liquor containing primarily, NaOH. The NaOH content of the liquor is consumed in pulping and the spent liquor, called black liquor, is combustible and contains lignin and residual chemicals. The black liquor is concentrated and then burned for its energy in a boiler, leaving a melt of combustion products consisting primarily of Na.sub.2 CO.sub.3. The melt is dissolved in water to form green liquor, which is reacted with lime in a slaker machine to produce Ca(OH).sub.2 which then reacts with the Na.sub.2 CO.sub.3 of the green liquor in the slaker machine to form NaOH. The NaOH can then be used in the digestion step described above.
The slaker apparatus or machine is a compact dual compartment machine. It consists of a slaking unit and a classifier compartment. The slaker unit is comprised of a steel cylindrical tank and an impeller type mixer supported by tank spanning beams and is designed to promote rapid intimate contact between lime and water additions. The rectangular tank classifier compartment is connected to the slaking unit by submerged slurry passage ports. It includes of a heavy duty conveyor means for lifting the grit along the ramp to the beach section above the liquid level in the classifier compartment, where the grit is washed, and then to a washed grit discharge conduit.
A slaker machine is one in which lime reacts with hot water in the slaking reaction as follows: EQU CaO+H.sub.2 O.fwdarw.Ca(OH).sub.2
In the slaking unit, lime and slaking liquid (water and weak liquor) are fed into the unit through inlets provided in the cover. Generally, a 3:1 liquid to lime ratio is used. The partially slaked slurry passes into the classifier where slaking continues for a time and the grit settles on the conveyor mechanism. The conveyor mechanism transports the grit up the ramp for a final water spray or wash and then to discharge for disposal. The degritted milk of lime from the clarified surface liquid layer in the classification compartment overflows into a launder which spans the classifier, and from the launder it is removed for utilization.
Of course, certain non-reactive materials including overburnt lime are introduced with the lime, and provision must be made for handling and disposing of these non-reactive materials as grit solids. Disposal of the grit solids has posed an environmental problem because the liquor associated with the grit is primarily the highly caustic product of the process; Ca(OH).sub.2. The pH of the grit-associated liquor has been measured at 13.2 to 13.5. A recent environmental standard has been established whereby solids disposed in landfills may not exceed a pH of 12.5. Spray washing of the grit material has not been found to be capable of reducing the pH to the extent necessary to respond to the new environmental requirement.
2. Description of Related Art
U.S. Pat. No. 4,853,115, issued Aug. 1, 1989 provides a general description of a classifier device of the reciprocating rake type.